TOPICAL REVIEW Electrowetting: from Basics to Applications

J. Phys. Condensed Matter: TOPICAL REVIEW Electrowetting: from Basics to Applications Frieder Mugele1,* and Jean-Christophe Baret1,2 1: University of Twente; Faculty of Science and Technology; Physics of Complex Fluids; P.O. Box 217; 7500 AE Enschede (The Netherlands) 2: Philips Research Laboratories Eindhoven; Health Care Devices and Instrumentation; WAG01; Prof. Holstlaan 4; 5656 AA Eindhoven (The Netherlands) *: corresponding author phone: ++31 / 53 489 3094; fax: ++31 / 53 489 1096; email: [email protected] 1 Abstract. Electrowetting has become one of the most widely used tools to manipulate tiny amount of liquids on surfaces. Applications range from lab-on-a-chip devices to adjustable lenses or new types of electronic displays. In the present article, we review the recent progress in this rapidly growing field including both fundamental and applied aspects. We compare the various approaches used to derive the basic electrowetting equation, which has been shown to be very reliable as long as the applied voltage is not too high. We discuss in detail the origin of the electrostatic forces that induce both the contact angle reduction as well as the motion of entire droplets. We examine the limitations of the electrowetting equation and present a variety of recent extensions to the theory that account for distortions of the liquid surface due to local electric fields, for the finite penetration depth of electric fields into the liquid, as well as for finite conductivity effects in the presence of AC voltage. The most prominent failure of the electrowetting equation, namely the saturation of the contact angle at high voltage, is discussed in a separate section. Recent work in this direction indicates that a variety of distinct physical effects - rather than a unique one – is responsible for the saturation phenomenon, depending on experimental details. In the presence of suitable electrode patterns or topographic structures on the substrate surface, variations of the contact angle can not only give rise to continuous changes of the droplet shape, but also to discontinuous morphological transitions between distinct liquid morphologies. The dynamics of electrowetting are discussed briefly. Finally, we give an overview of recent work aimed at commercial applications, in particular in the fields of adjustable lenses, display technology, fiber optics, and biotechnology-related microfluidic devices. 2 1. Introduction Miniaturization has been a technological trend for several decades. What started out initially in the microelectronics industry has long reached the area of mechanical engineering, including fluid mechanics. Reducing size has been shown to allow for integration and automation of many processes on a single device giving rise to a tremendous performance increase, e.g. in terms of precision, throughput, and functionality. One prominent example from the area of fluid mechanics are Lab-on-a- Chip systems for applications such as DNA- or protein analysis, and biomedical diagnostics [1-3]. Most of the devices developed so far are based on continuous flow through closed channels that are either etched into hard solids such as silicon or glass, or replicated from a hard master into a soft polymeric matrix. Recently, devices based on the manipulation of individual droplets with volumes in the range of nanoliters or less have attracted increasing attention [4-10]. From a fundamental perspective the most important consequence of miniaturization is a tremendous increase in the surface-to-volume ratio, which makes the control of surfaces and surface energies one of the most important challenges both in microtechnology in general as well as in microfluidcs. For liquid droplets of submillimeter dimensions, capillary forces dominate [11, 12]. The control of interfacial energies has therefore become an important strategy to manipulate droplets at surfaces [13-17]. Both liquid-vapor and solid-liquid interfaces have been influenced in order to control droplets, as recently reviewed by Darhuber and Troian [15]. Temperature gradients as well as gradients in the concentration of surfactants across droplets give rise to gradients in interfacial energies, mainly at the liquid-vapor interface, and thus produce forces that can propel droplets making use of the thermocapillary and Marangoni effects. Chemical and topographical structuring of surfaces has received even more attention. Compared to local heating, both of these two approaches offer much finer control of the equilibrium morphology. The local wettability and the substrate topography together provide boundary conditions within which the droplets adjust their morphology to reach the most energetically favorable configuration. For complex surface patterns, however, this is not always possible as several metastable morphologies may exist. This 3 can lead to rather abrupt changes in the droplet shape, so-called morphological transitions, when the liquid is forced to switch from one family of morphologies to another by varying a control parameter, such as the wettability or the liquid volume [13, 16, 18-20]. The main disadvantage of chemical and topographical patterns is their static nature, which prevents active control of the liquids. Considerable work has been devoted to the development of surfaces with controllable wettability – typically coated by self- assembled monolayers. Notwithstanding some progress, the degree of switchability, the switching speed, the long-term reliability, and the compatibility with variable environments that have been achieved so far are not suitable for most practical applications. In contrast, electrowetting (EW) has proven very successful in all these respects: contact angle variations of several tens of degrees are routinely achieved. Switching speeds are limited (typically to several milliseconds) by the hydrodynamic response of the droplet rather than the actual switching of the equilibrium value of the contact angle. Hundreds of thousands of switching cycles were performed in long term stability tests without noticeable degradation [21, 22]. Nowadays, droplets can be moved along freely programmable paths on surfaces, they can be split, merged and mixed with a high degree of flexibility. Most of these results were achieved within the past five years by a steadily growing community of researchers in the field [23]. Electrocapillarity, the basis of modern electrowetting, was first described in detail in 1875 by Gabriel Lippmann [24]. This ingenious physicist, who won the Noble prize in 1908 for the discovery of the first color photography method, found that the capillary depression of mercury in contact with electrolyte solutions could be varied by applying a voltage between the mercury and electrolyte. He formulated not only a theory of the electrocapillary effect but developed several applications, including a very sensitive electrometer and a motor based on his observations. In order to make his fascinating work, which has only been available in French up to now, available to a broader readership, we included a translation of his work in the Appendix of this review. The work of Lippmann and of those who followed him in the following more than hundred years was devoted to aqueous electrolytes in direct contact with mercury surfaces or mercury droplets in contact with insulators. A major obstacle to broader applications was 4 electrolytic decomposition of water upon applying voltages beyond a few hundred millivolts. The recent developments were initiated by Berge [25] in the early 1990s, who introduced the idea of using a thin insulating layer to separate the conductive liquid from the metallic electrode in order to eliminate the problem of electrolysis. This is the concept that has also become known as electrowetting on dielectric (EWOD). In the present review, we are going to give an overview of the recent developments in electrowetting, touching only briefly on some of the early activities that were already described in a short review by Quilliet and Berge [26]. The article is organized as follows: in section 2 we discuss the theoretical background of electrowetting, comparing different fundamental approaches, and present some extensions of the classical models. Section 3 is devoted to materials issues. In section 4, we discuss the phenomenon of contact angle saturation, which has probably been the most fundamental challenge in electrowetting for some time. Section 5 is devoted to the fundamental principles of electrowetting on complex surfaces, which is the basis for most applications. Section 6 deals with some aspects of dynamic electrowetting, and finally, before concluding, a variety of current applications ranging from lab-on-a-chip to lens systems and display technology are presented in section 7. 2. Theoretical Background Electrowetting has been studied by researchers from various fields, such as applied physics, physical chemistry, electrochemistry, and electrical engineering. Given the various backgrounds, different approaches were used to describe the electrowetting phenomenon, i.e. to determine the dependence of the contact angle on the applied voltage. In this chapter, we will – after a few introductory remarks about wetting in section 2.1 – discuss the main approaches of electrowetting theory (sec. 2.2): the classical thermodynamic approach (2.2.1), the energy minimization approach (2.2.2), and the electromechanical approach (2.2.3). In section 2.3, we will describe some extensions of the basic theories that give more insight into the

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